While I'm sure most Quicksilver users opt to remove its icon from the Dock, so it doesn't show up when using Command-Tab, having it visible in the Dock is great for drag 'n' drop operations. Luckily we can have the best of both worlds.
In the Quicksilver preferences, uncheck the 'show icon in dock' option (requires restart). After relaunching it, drag the icon from your Applications folder into the Dock. Quicksilver's icon will remain there, albeit without the black triangle indicating it has been launched. Notice that Quicksilver does not appear in the Application switcher (Command-Tab), however you can still perform drag 'n' drop operations.
This hint presumably works for any application that lets you run it without a Dock icon.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2007020808174741